CHROMO-NITRIC ACID. 29 



at one time employed solutions of 40 to 50 per cent, for the ova of inverte- 

 brates. This of course has the advantage of a very rapid fixing action. 

 . Nitric acid has the valuable property of hardening yolk without making 

 it brittle. But, for general purposes at all events, the pure nitric acid solu- 

 tions may be considered to be superseded by Perenyi's chromo-nitric acid 

 mixture (below, 39). 



38. Nitrate of Silver has been used for fixation, but is too 

 uncertain in its action to be recommendable. 



39. Chromo-nitric Acid (PERENYI'S formula, Zool. Anzeig., 

 v, 1882, p. 459). 



4 parts 10 per cent, nitric acid. 



3 parts alcohol. 



3 parts 0'5 per cent, chromic acid. 



These are mixed, and after a short time give a fine violet- 

 coloured solution. 



The objects are immersed for four to five hours, and then 

 passed through 70 per cent, alcohol (twenty- four hours) strong 

 alcohol (some days), absolute alcohol (four to five days). 

 They are then fit for cutting. The advantage of the process 

 is, amongst others, that segmentation spheres and nuclei are 

 perfectly fixed, the ova do not become porous, and cut like 

 cartilage. 



Chromo-nitric acid is not only an embryological reagent, 

 and a very important one, but also an admirable one for 

 general work. I have found it altogether excellent for pre- 

 serving marine organisms, especially large forms. Strong 

 alcohol need only be used if the objects are destined to be 

 sectioned. 



Another advantage is that the fixing solution may be combined with a 

 stain. (In this case the albuminous envelopes of the ova must be carefully 

 removed, otherwise the stain will not penetrate.) 



Some stains, such as fuchsin or anilin red, may be dissolved directly in 

 the fixing solution. Others, such as eosin, purpurin, anilin violet, must first 

 be " dissolved in three parts of alcohol, and then shaken into the liquid." 



Picro-carmine and borax-carmine may be added to the liquid, but they 

 give rise to a precipitate, which must be removed by filtration before using. 



Another formula given by Perenyi (Zool. Anzeig., 274, 1888, p. 139, and 

 276, p. 196) is as follows : 



3 parts 20 per cent, nitric acid. 



3 parts 1 per cent, chromic acid. 



4 parts absolute alcohol. 



For embryos of Lacerta. Fix for twenty minutes. Wash out for an hour 

 with 70 per cent, alcohol, and then with strong alcohol. Stain with Dela- 



